The responsibility isn't on you (4)
I believe the answer is:
onus
'the responsibility' is the definition.
(onus is a duty or responsibility)
'isn't on you' is the wordplay.
'isn't' becomes 'ons' (I can't justify this - if you can you should give a lot more credence to this answer).
'on' indicates putting letters inside (as in clothing 'on' a person).
'you' becomes 'u' (commonly used in online communication).
'ons' enclosing 'u' is 'ONUS'.
Can you help me to learn more?
(Other definitions for onus that I've seen before include "Capulet opponent (R&J)" , "'Responsibility, task or burden (4)'" , "'Burden, of proof (4)'" , "Burden, duty" , "Liability" .)
